First of all if the car was running well for three days how could you have put Diesel Fuel into the car? The car company is trying to blame you for a maintenance flaw the car had before.

Cars like any human can get sick and not work. The car rental company should be compensating you for the lost time on the road, for your safety. What if the car would have caught on fire who would be responsible then?

My point is you have insurance and as others are saying you used your credit card so let them fight for you.

I would call the rental company and dispute the claim. 5 weeks is quite a long time to call you to get money. What if they were able to rent it out right after your rental? My point is how can they prove it was you who damaged the car?

You called the Roadside Help number and this was your only obligation. You were in the middle of nowhere and your car broke down. They signed in the car that was not working and swapped you for a new car.

When you returned your second car there was no mention of paying for your first car. Once you signed in your first car by giving it to the driver of your second car you released yourself from all liabilities. The driver did not make any notes that would make you responsible for any damage.

Your Loss And Damage Policy, Loss and Damage Policy and Personal Accident Insurance from your credit card should cover everything. I would dispute any charges because there is no proof.

When you get the letter I would send them a letter asking for evidence. Whose to say the renter before or after you did something with the car.

It sounds fishy that you put Diesel fuel in the car as the nozzle won't even fit. You put Premium Gas and as Puerto Rico is a US Territory your defense is you in good faith filed up the car with Premium Unleaded Fuel. If they were so strict about how you filled up the car they should have given you tips on which types of gas to use and how the nozzle/machine looks.

You the renter can't be expected to know every damn little thing about the car. I mean come on there are many different petrol stations throughout the world and the machines are in different languages. I think the car companies should be more proactive showing you how the car works, what gas to use etc.

Please keep us posted on what comes of this. I would not pay any demand Remember they are sending you a letter with the price for the damage without any evidence. It's considered a Civil Recovery Demand. Unless they press charges in court(which I doubt) because it would take them time I would not pay the fee and write a letter stating that you do not believe you are responsible. If you intentionally broke a window or abandoned the car at the airport terminal then you would be responsible.

I would ask for free legal advice on Expertlaw.com. This web forum has free advice from volunteers many of them who are lawyers. I have used this website and it has been very helpful to me.

Good luck and hope it works out in your favor.